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81.
It is shown by use of Mössbauer spectroscopy that the spin-spin relaxation time of Fe3+ ions in frozen aqueous solutions is increased when two different iron complexes are present. 相似文献
82.
Prof. Torgeir Nakken Kenneth D. Knudsen Arne Mikkelsen Arnljot Elgsaeter 《Rheologica Acta》1995,34(3):235-247
High-Q torsional resonators constitute the most sensitive transducers for high frequency dynamic viscoelastic measurements of dilute polymer solutions. Most such resonators described in the literature are segmented. Because of the need for torque and torsional displacement transducers the Q-value of the individual segments most often differ, but normally all segments have the same radius.A detailed analysis of the dynamics of such resonators when both the radii, material properties and surrounding media may be different for each segment, is presented. For resonators where all segment lengths equal an integer multiple of a quarter of the torsional wavelength, we find that the Q-value of the resonator as a whole is mainly determined by the Q-value of the segment with the smallest radius. We further find that reduction of the radius of the segment surrounded by polymer solution results in a stronger mechanical coupling between the resonator as a whole and the polymer solution. These findings suggest that the segment radii are important optimization parameters of segmented torsional resonators used to measure the high frequency dynamic viscoelastic properties of e.g. polymer solutions. 相似文献
83.
An autonomous fourth order model of vocal fold vibrations is proposed. Each fold is represented by a lower and upper mass, and the aerodynamic forces are derived from a modified Bernoulli equation. The model exhibits many features of normal phonation in a wide parameter region. At the borderlines of this region coexistence of limit cycles, period-doubling and chaos are observed. Implications for an understanding of pathological voices are discussed. 相似文献
84.
Hakon H. Sigurdsson Elin Knudsen Thorsteinn Loftsson Nigel Leeves Johanna F. Sigurjonsdottir Már Másson 《Journal of inclusion phenomena and macrocyclic chemistry》2002,44(1-4):169-172
In this study the interactions between a cationic polymer and an anionic cyclodextrin were investigated. The system has the potential for use in a sustained release dosage forms for use on mucous membranes. As mucous membranes are negatively charged the objective of this study was to investigate whether a drug delivery system based on a cationic polymer and an anionic cyclodextrin would be more mucoadhesive than a system containing a cationic polymer and a neutral cyclodextrin. For this purpose the cationic polymer hexadimethrine bromide (HDMBr) and anionic sulfobutylether -cyclodextrin (SBECD) were utilized as well as the neutral hydroxypropyl-cyclodextrin (HPCD). Triclosan was used as a model drug. The drug delivery system was formulated as a solution or semi-solid and its adhesion to porcine buccal mucosa and cation exchange media was measured. In addition the release of triclosan from the system was quantified. No difference was observed between the two systems when they were applied to the mucosal surface. However, the formulations showed improved adhesion, compared to the neutral cyclodextrin/drug delivery system, when they could also reach the underlying surface of the excised tissue. The drug delivery system was much better retained on the cation exchange media than the uncharged system. Significant interactions were observed between the negatively charged cyclodextrin and the positively charged polymer. The results indicate that the interactions could be used to obtain a mucoadhesive sustained drug delivery system under certain circumstances. The positive charge of HDMBr did not have the expected effect on the buccal mucosa and it can be concluded that although a positive charge is likely to promote mucoadhesion, other attributes of polymers, such as molecular weight and viscosity, may have equally beneficial effect. 相似文献
85.
Attempts are made to build a bridge between asymmetric catalysis and enzymatic reactions by mechanistic investigations and the development of a catalytic and enantioselective approach to amination of alpha-keto esters by primary amines catalyzed by chiral Lewis acids as a model for transamination enzymes. Different Lewis acids can catalyze the half-transamination of alpha-keto esters using primary amine nitrogen sources such as pyridoxamine and 4-picolylamine. The mechanistic studies of the Lewis-acid catalyzed half-transamination using deuterium-labelled compounds show the incorporation of deuterium atoms in several positions of the alpha-amino acid derivative, indicating that the enol of the alpha-keto ester plays an important role along the reaction path. The catalytic enantioselective reactions are dependent on the pKa-value of the solvent since enantioselectivities were only obtained in solvents with high pKa-values relative to methanol. However, stronger acidic conditions generally gave better yields, but poor enantioselectivities. A series of chiral Lewis acids were screened as catalysts for the enantioselective half-transamination reactions and moderate yields and enantioselectivities of up to 46% ee were obtained. 相似文献
86.
Deb SK Davis B Knudsen GM Gudihal R Ben-Amotz D Davisson VJ 《Journal of the American Chemical Society》2008,130(30):9624-9625
Accurate quantification of protein content and composition has been achieved using isotope-edited surface enhanced resonance Raman spectroscopy. Synthesis of isotopomeric Rhodamine dye-linked bioconjugation reagents enabled direct labeling of surface lysines on a variety of proteins. When separated in polyacrylamide gels and stained with silver nanoparticles. The spectral signatures reflect the expected statistical distribution of isotopomeric labels on the labeled proteins in the gel matrix format without interference from protein features. 相似文献
87.
Søndergaard K Kristensen JL Palner M Gillings N Knudsen GM Roth BL Begtrup M 《Organic & biomolecular chemistry》2005,3(22):4077-4081
From codeine, four different 2-aryl substituted apomorphines were synthesised in 6 steps each. Oxidation of codeine with IBX followed by acid catalysed rearrangement gave morphothebaine, which was selectively triflylated at the 2-position and subsequently O-acetylated at the 11-position. The resulting triflate was coupled in a Suzuki-Miyaura type reaction with a series of 4-substituted arylboronic esters which, after deprotection, gave the desired 2-aryl apomorphines. The analogues were tested for affinity towards a range of dopaminergic, serotonergic and adrenergic receptors. 2-(4-Hydroxyphenyl)-apomorphine exhibited high affinity for the dopamine D2 receptor. A putative ligand-receptor interaction was put forward. 相似文献
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